The microstructures (either conductive or insulating) are commonly used in a myriad of applications. A typical example is a circuitized substrate for packaging a chip of semiconductor material (wherein an integrated circuit is formed); generally, the substrate is made of an insulating base for supporting conductive connection elements. For instance, in an electronic device of the Ball Grid Array (BGA) type, the chip is mounted on an upper surface of the substrate. Terminals of the chip are then bonded to corresponding contacts (such as pads), which are provided on the same upper surface of the substrate (such as with a flip-chip technique). Through via-holes (or simply vias) cross the substrate, so as to connect the pads on its upper surface with corresponding conductive balls provided on a lower surface of the substrate. The structure so obtained is then enclosed within a protective cover (for example, of plastic material), which only leaves exposed the balls on the lower surface of the substrate (implementing external terminals of the electronic device).
In this context, a problem is due to the demand of higher and higher density of the contacts for the terminals of the chips; indeed, the growing complexity of the integrated circuits requires a huge number of terminals in the chips (especially for multiprocessor components), with a corresponding decrease of their pitch. However, even very sophisticated production processes—such as the ones based on the micro-via technology, also known as Sequential Build-Up (SBU) or High Density Interconnect (HDI)—do not allow obtaining vias with a diameter smaller than 50-80 μm.
The increasing terminal density of the chips also raises a big issue for the dissipation of heat that is produced by the chips; this may be very critical, especially at specific areas (hot-spots) where the heating of the chips concentrates.
Another problem of the know substrates is the requirement of very low resistance of the connections between the terminals of the chips and the terminals of the electronic device through the corresponding vias. For example, in complex System on Chips (SOCs), such as for mobile telephones wherein almost all the functions thereof are implemented in a few chips, the resistance of each connection should not exceed 70-80 mΩ.
On the other hand, in some applications (for example, when the electronic devices work at high frequencies) a low dielectric constant of the insulating base of the substrate is required, in order to reduce the coupling stray capacitance of the connections. Moreover, in power applications it is also required that the insulating base should exhibit a high breakdown voltage.
A further problem concerns the reliability of the electronic devices. Indeed, any difference between the Thermal Coefficient of Expansion (TCE) of the chips and of the substrates may cause mechanical stresses on the chips (especially at their bonding with the substrates in the flip-chip technique). The problem is further exacerbated by the use of ultra-low-k dielectric materials, which demand that the substrates should virtually cause no stress on the chips.
Similar considerations apply to Package-On-Package (POP) structures, wherein two or more electronic devices (each one with the corresponding substrate) are mounted one above the other.
Another example of application of the microstructures is in Micro-ElectroMechanical Systems (MEMS). In this case, it is required the capability of making (conductive or insulating) microstructures of complex shape; desired characteristics of these microstructures may be high mechanical stiffness, low weight and/or resistance to extreme temperatures (for example, down to −100° C. or up to +1,500° C.).
Moreover, the microstructures may also be used in precision micro-mechanical applications (for example, in watches). This may require (conductive or insulating) microstructures very stable with respect to the temperature, with low inertia and/or suitable to be used in harsh environments (for example, being corrosion-resistant).
Equivalent, additional and/or different problems are also experienced in other technical fields that require the use of microstructures. For example, this is the case of magneto-dielectric materials (such as for use in electromagnetic antennas), vacuum electronic devices (such as microware tubes, or better in the TeraHz frequency region considering the dimensions of the microstructures), and so on.